Conventionally, in a communication device such as a mobile phone and the like, a filter device has been used to separate signals having different bands, such as transmission and reception signals, for example. In such a filter device, one or more surface acoustic wave (SAW) elements are often used. These SAW elements can include one or more resonators, each formed by an interdigital transducer (IDT) electrode disposed on a piezoelectric substrate. The IDT electrode includes two comb-shaped electrodes that interdigitate with one another. In particular, such SAW elements also including a silicon dioxide film covering a piezoelectric substrate made of lithium niobate or lithium tantalate have been used in the development and design of SAW filters and duplexers for application to the cellular handset market. The piezoelectric substrate and the silicon dioxide film cooperate with each other for temperature compensation to provide better temperature characteristics for the SAW elements.
The acoustic modes present in such IDT-based SAW elements include Lamb waves that propagate in the silicon dioxide film. These Lamb waves can cause spuriousness in the frequency characteristics of the SAW element. It has been analytically proven that characteristics of the Lamb waves, such as resonant frequency, coupling, etc., are dependent on the thickness of the silicon dioxide film. In particular, it has been shown (see, for example, Ben Abbott, Robert Aigner, Alan Chen, Kevin Gamble, Julien Gratier, Taeho Kook, Marc Solal, Kurt Steiner, “THEORETICAL INVESTIGATION INTO SPURIOUS MODES CONTENT IN SAW DEVICES WITH A DIELECTRIC OVERCOAT,” Fourth International Symposium on Acoustic Wave Devices for Future Mobile Communication Systems, March 3-5, 2010, pp. 193-203) that to suppress the Lamb waves the thickness of the silicon dioxide film must be restricted to less than a certain amount. However, this requirement can be inconsistent with other design considerations for various SAW elements.
For example, certain SAW filters can have a ladder-type configuration including a plurality of series-arm resonators and a plurality of parallel-arm resonators, as shown in FIG. 1, for example, and discussed further below. One technique for suppressing suppress frequency fluctuations in the characteristics of one or more of the series-arm resonators is to increase the film thickness of a dielectric film formed on an area of the series-arm resonator(s), as disclosed in U.S. Patent Application Publication No. 2013/0162368, for example. Another technique involves changing the thicknesses of the dielectric films over the series-arm resonator(s) of different stages of a SAW filter, as disclosed in Japanese Patent Publication No. 2000-068784, for example. Further, U.S. Patent Application Publication No. 2015/0270824 discloses a technique for separating the frequency of a secondary mode in an electroacoustic device from the frequency of the main mode by changing the film thicknesses of a separating film in the device.
FIG. 1 shows an example of conventional filter device 100 formed by SAW elements. The filter device 100 includes a first filter 110 having a first passband and a second filter 120 having a second passband different from the first passband. The first filter 110 is connected between a common contact 101, which can be connected to an antenna, and a first signal contact 102. The second filter 120 is connected between the common contact 101 and a second signal contact 103. In the first filter 110, series resonators 111, 112, 113 and parallel resonators 114, 115 form a ladder-type filter. Similarly, in the second filter 120, series resonators 121, 122, 123 and parallel resonators 124, 125 form another ladder-type filter. Each of these series resonators 111, 112, 113, 121, 122, 123 and parallel resonators 114, 115, 124, 125 is a SAW element.
FIG. 2 is a graph illustrating signal reflectivity (as a function of normalized frequency) of the first conventional filter 110 presented at the common contact 101. The first filter 110 has a passband 132. In the first filter 110, a spurious emission 136 may appear in a band 134 that includes the frequency range 1.2 to 1.4 times greater than the center frequency f0 of the passband 132. The spurious emission 136 results from a Lamb wave occurring in the silicon dioxide film of the SAW element forming the first filter 110.